Improving HTTP/2 Race-Condition Synchronization Through TCP Segment Reordering

Single-packet synchronization techniques for HTTP/2race-condition testing have been shown to reduce sensi-tivity to network jitter. However, their effectivenessremains influenced by transport-layer fragmentation be-havior in real-world TCP/IP stacks.Building upon Ryotak’s First Sequence Sync tech-nique, we extend the underlying transport-layersynchronization concept to TLS-enabled HTTP/2 en-vironments. We present a mechanism that selectivelydelays and reorders TCP segments during transmis-sion, providing finer control over the arrival timing offragmented HTTP/2 request payloads without requiringmodifications to the application-layer protocol.We implement and evaluate our approach in a dis-tributed cloud testbed using geographically separatedattacker and server instances. Across 50 experimen-tal runs involving 5,000 concurrent requests per run,our method reduced the interval between the first andlast request processed by the server by approximately4.64× relative to a standard last-byte synchronizationbaseline. The mean synchronization window decreasedfrom 263.8 ms to 56.8 ms, corresponding to a 78.5%reduction.Our results suggest that transport-layer packet order-ing can significantly influence synchronization preci-sion in HTTP/2 race-condition experiments and mayimprove the reproducibility of timing-sensitive securityevaluations. Github repository: https://github.com/0-3-5/tcp-reordering-synchronization#tcp-reordering-synchronization